The present disclosure relates to placing fill materials into the holes of a substrate, such as placing an electrically conductive, thermally conductive or nonconductive paste into holes within printed circuit boards (PCB), wired circuit boards, hybrid circuit boards, ceramic substrates, and other various laminate and electronic packages and panels.
Vias, holes, or openings are common structures found in PCBs and other electronic panels. The intent is to uniformly fill the plurality of vias or holes on a substrate with a desired fill material.
However, a common issue with the current application systems is the undesirable levels of fill variation, including the lack of fill material in some holes and excessive fill material in others, resulting in substantial material waste. Especially with squeegee print filling systems and processes, fill material flow is often non-uniformly restricted in random holes.
In other application systems, an air pressure action is used to force paste from a paste tube through a dispenser. The dispenser moves across a substrate or panel while dispensing the paste into holes within the panel. These application systems may yield potential defects caused by the preparation of the materials or by the application method itself. With these application systems, it is difficult to reliably place paste in a hole without forming an air pocket or void. This is partially due to variations in the substrate (thickness, hole diameter to be filled, density of holes in any given area across the panel). Fluctuations in the air pressure pushing the fill material also result in erratic rates of paste flow through the dispenser. Another problem is that the paste or fill material within a tube may be introducing variations in viscosity and flow, resulting in uneven application, inadequate and/or excessive fill. For example, the viscosity of the paste in the center of the tube appears to be different than the viscosity of the paste in the beginning of the tube. There is also a concern that the use of air pressure may penetrate, blow-by, or compromise the seal in the paste tube and create air voids in the paste supply.
These issues combine to produce poor quality fill (voiding) within the via or hole and result in an excessive amount of fill material waste and expense while attempting to adequately fill the substrate holes. There can also be excess paste under a leading edge of the dispenser that typically cannot be reused. The excess paste is usually cured and discarded, as there is a potential for paste contamination caused by O-ring debris, catch pans, etc. The excessive material waste creates a need to constantly add more paste to the application systems to replenish the paste volume in the dispenser.
It is important that the vias or holes must be completely filled with paste so that there are no air pockets. If there are voids or air pockets in the paste, these air pockets generally remain in the completed product. A via with a void has several adverse effects. If the paste is placed to provide thermal conductivity, the air of the void acts as an insulator. If the paste is placed to provide electrical conductivity, an opening at the void results in no secondary or fail safe electrical connection being formed. Furthermore, if the via is filled to provide structural integrity, a void in the via results in reduced structural integrity.
Thus there is a need for an improvement to the current hole fill application systems. There is a need for a process and apparatus for uniformly placing fill material with minimal waste into the holes of an electronic substrate such that there are no air pockets formed in the plugged fill material. The process and apparatus needs to be able to form plugged holes in a substrate which have reliable electrical conductivity, thermal characteristics, and/or structural integrity. There is a need for a process and apparatus that improves the yield for forming plugged holes in electronic substrates while lessening the possibility that contaminates will be introduced into the fill material. Furthermore, there is a need for a hole fill process which is controllable and which has a high throughput, such as in a relatively high speed, single pass operation. There is also a need for a effective and time efficient cleaning process and apparatus to clean the hole fill apparatus with minimal disassembly.